Controller for wire electric discharge machine

ABSTRACT

A discharge pulse number generated between a wire electrode and a workpiece is counted every predetermined time. A ratio Px/Ps of this counted value Px to the reference pulse number Ps will be determined to control an amount of coolant in response to this ratio Px/Ps. Also, in response to this ratio Px/Ps, an amount of movement within predetermined time is controlled. Further, through the ratio Px/Ps and the like, quiescent time to be controlled by the detection voltage generator is controlled. Thereby, surplus supply of energy is prevented, the machining speed and machining precision is improved and any disconnection of the wire electrode is avoided.

CORSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/023,906 filed Dec. 21, 2001, which claims the benefit of JapanesePatent Application No. 2000-391748 filed Dec. 25, 2000. The disclosuresof the foregoing applications are all incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a controller for a wire electricdischarge machine, and more particularly to a control method capable ofimproving the machining speed and machining precision.

2. Description of the Prior Art

FIG. 18 is a view showing the outline of a conventional controller forwire electric discharge machine. A discharge pulse generator 1 appliesvoltage to a gap between a wire electrode 4 and a workpiece 5 in orderto perform electric discharge machining, and is constituted by a DCpower source, a circuit composed of a switching element such as atransistor, a charge/discharge circuit of a capacitor and the like.

A detection voltage generator 2 is a device for applying pulse voltagebetween the wire electrode 4 and the workpiece 5 in order to detectwhether or not discharge can be performed at the gap between the wireelectrode 4 and the workpiece 5, is constituted by an active elementsuch as a transistor, a circuit composed of a resistor, a capacitor andthe like, a DC power source and the like.

A current-supply brush 3 is used to supply a wire electrode withcurrent, and is connected to one terminal of the discharge pulsegenerator 1 and one terminal of the detection voltage generator 2respectively. Also, the workpiece 5 is connected to the other terminalof the discharge pulse generator 1 and the other terminal of thedetection voltage generator 2 respectively. Between the wire electrode 4traveling and the workpiece 5, there is applied pulse voltage to begenerated from the discharge pulse generator 1 and the detection voltagegenerator 2.

A discharge gap detection device 6 is connected to the workpiece 5 andthe wire electrode 4, and judges on the basis of the transition ofdetection pulse voltage from the detection voltage generator 2 whetheror not the discharging gap is in a dischargeable state. Ifdischargeable, the discharge gap detection device 6 generates adischarge pulse supply signal. Further, output from the discharge gapdetection device 6 is processed by an equalizing circuit 22, andthereafter, is compared with output from a reference voltage settingdevice 23 to thereby obtain voltage deviation. The voltage deviation isinputted into a feed pulse arithmetic unit 24 in order to controlfeeding of the wire electrode. The output (pulse-like gap voltage ofseveral μ seconds to several tens μ seconds) from the discharge gapdetection device 6 is processed by the equalizing circuit 22 in order tomatch the output to the processing speed of the feed pulse arithmeticunit 24.

The feed pulse arithmetic unit 24 generates a pulse train, that controlsthe feed pulse space, on the basis of the voltage deviation to output toa feed pulse distribution device 12. The feed pulse distribution device12 distributes this pulse train to driving pulses for X-axis and Y-axisin accordance with a machining program to output to a X-axis motorcontroller 10 and Y-axis motor controller 11 which drive a table withthe workpiece 5 mounted thereon.

First, in order to detect whether or not discharge can be performedbetween the workpiece 5 and the wire electrode 4, detection pulsevoltage is caused to be generated from the detection voltage generator 2to apply to a gap between the workpiece 5 and the wire electrode 4.Then, a current is passed through between the workpiece 5 and the wireelectrode 4. Then, when a voltage drop occurs between the workpiece 5and the wire electrode 4, the discharge gap detection device 6 detectsthis voltage drop to judge it to be dischargeable, and transmits adischarge pulse supply signal to the discharge pulse generator 1.

As a result, the discharge pulse generator 1 generates a discharge pulseto supply the gap between the workpiece 5 and the wire electrode 4 withdischarge pulse current. Thereafter, after the elapse of appropriatequiescent time during which the gap is cooled, the detection pulse willbe applied to the gap again. The above described operation cycle will berepeatedly performed for electric discharge machining.

As described above, machining to remove one portion from the workpiece 5is performed every time the discharge pulse occurs. More specifically,through the use of the detection pulse voltage, there is searched for aminute conductive passage of several tens μm or less to be formed in agap between the wire electrode 4 and the workpiece 5, which are oppositeto each other, and the discharge pulse current is flowed immediately forheating, transpiration or melting and splashing to thereby startdischarging. An amount of removal per discharge pulse and machiningperformance differ dependent on magnitude of the discharge pulsecurrent, characteristics such as heat of fusion, coefficient of thermalconductivity and viscosity during melting of materials of the wireelectrode 4 and the workpiece 5, characteristics relating to cooling dueto coolant (machining liquid) and sludge discharge, and the like.

Also, the next discharge subsequent to the generation of a certaindischarge tends to concentratedly occur in the vicinity of a place wherethere exist a multiplicity of micro conductive passages through sludgewhich is mainly generated immediately after the previous discharge isterminated. For this reason, precise servo feed control and quiescenttime control are requested so as to prevent discharges which occur oneafter another from being concentrated on one place.

FIG. 19 is a view showing a monitored waveform for machining voltage,machining current and machining speed when a square pillar of die steelshown in FIG. 13 has been cut out by a conventional method. At a cornerwhere the direction of machining changes by right angle, idle feeding isperformed by an amount corresponding to the gap in an instant. For thisreason, the machining current decreases and the machining voltagebecomes higher. Accordingly, the feed speed command becomes larger.After the direction is changed, the wire electrode and the workpieceapproach to each other more than necessary to make the gap narrower, andit becomes difficult to discharge sludge smoothly. As a result, thesludge concentration becomes higher to cause discharge concentration,resulting in short-circuit or disconnection of the wire electrode.

For this reason, in the conventional control, it is necessary to provideservo feed (relative feed of the wire electrode to the workpiece) ordischarge quiescent time within a fixed time or distance immediatelyafter the corner, and further to separately add a process for confirmingliquid pressure of the coolant in advance for evaluation. Moreover,precision correction to cope with change in plate thickness and shape ofthe corner of the workpiece is very complicated and difficult.

In the conventional feed control, as described above, since thedetection is performed in terms of gap voltage, it lacks accuracy offeed, and disconnection easily occurs particularly in a state in whichthe wire electrode has been stretched tight, and therefore, it is notpossible to comply with any desire to improve the machining speed. Also,particularly at a corner portion or the like in the shape of machining,disconnection easily occurs, and in order to prevent the disconnection,it is necessary to add a corner control process and the like such asdecreasing the feed speed or the machining current.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a controller for awire electric discharge machine capable of wire electric dischargemachining at predetermined feed speed and machining current without anydisconnection of a wire electrode particularly on machining a cornerportion and the like.

With the controller for a wire electric discharge machine according tothe present invention, electric discharge machining is performed byapplying electric discharge pulse current between a wire electrode and aworkpiece while a wire electric discharge machine is causing the wireelectrode and the workpiece to relatively move to each other.

A first aspect of the controller has: discharge pulse number countingmeans for counting a discharge pulse number applied every predeterminedtime, or discharge pulse current integrated value computing means forcomputing to integrate discharge pulse current applied; moving means forrelatively moving the wire electrode and the workpiece to each otheralong a machining path on the basis of a moving command; and memorymeans for storing a discharge pulse number which is used as a referenceor a reference discharge pulse current integrated value; and determinesa ratio of a numerical value obtained by the discharge pulse numbercounting means or the discharge pulse current integrated value computingmeans to the reference discharge pulse number or the integrated value,and outputs distance obtained by multiplying relative moving distancebetween the wire electrode and the workpiece to be determined by thepreset feed speed and the predetermined time by the above describedratio, to the moving means as a moving command every the predeterminedtime.

Also, there is provided means for controlling a discharge quiescent timesuch that a discharge pulse number for every predetermined timecoincides with a reference discharge pulse number in response to thedischarge pulse number for every predetermined time and the referencedischarge pulse number, or there is provided means for controlling adischarge quiescent time such that a discharge pulse number for everypredetermined time coincides with a reference discharge pulse number inresponse to the discharge pulse current integrated value for everypredetermined time and the reference discharge pulse current integratedvalue, to thereby prevent surplus energy from being supplied.

Furthermore, there is provided a liquid amount controller whichincreases or decreases an amount of coolant on the basis of a ratio of adischarge pulse number for every predetermined time or the dischargepulse current integrated value to a reference discharge pulse number ora reference integrated value to control the amount of the coolant.

Also, a second aspect of the controller is to control a relative movingdistance of a wire electrode with respect to a workpiece by means ofmoving distance control means on the basis of an amount of machining ofthe workpiece through electric discharge pulse, or to control adischarge quiescent time, or to control an amount of coolant.

The control based on an amount of machining of a workpiece throughdischarge pulses detects an amount of discharge machining on the basisof a counted value obtained by counting discharge pulse number appliedevery predetermined time and a predetermined value determined inadvance, or detects an amount of discharge machining on the basis of avalue obtained by computing to integrate discharge pulse current appliedevery predetermined time and a predetermined value determined inadvance.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further objects and features of the present invention willbecome apparent from the following description of embodiments withreference to the accompanying drawings, in which:

FIG. 1 is an essential block diagram showing a controller for a wireelectric discharge machine according to a first embodiment of thepresent invention;

FIG. 2 is an essential block diagram showing a controller for a wireelectric discharge machine according to a second embodiment of thepresent invention;

FIG. 3 is a view for explaining generation of an amount of movementcaused by a change in the amount of machining;

FIG. 4 is an explanatory view for illustrating an amount of movementcorresponding to the change in the amount of machining;

FIGS. 5A to 5C are views showing that relationship between the amount ofmovement and the discharge pulse number varies depending upon materialof the workpiece, plate thickness thereof, machining groove width andthe like;

FIGS. 6A and 6B are views for explaining relationship between sludgeconcentration and average machining voltage, discharge pulse number andactual quiescent time;

FIGS. 7A and 7B are views for explaining relationship between actualquiescent time and discharge pulse number and average machining voltage;

FIG. 8 is a view for explaining relationship between quiescent time andan amount of machining and the like;

FIG. 9 is a view for explaining quiescent time control at point A inFIG. 7A;

FIG. 10 is a view for explaining quiescent time control at point C inFIG. 7B;

FIG. 11 is a view for explaining control of a liquid amount due to achange in an amount of machining;

FIG. 12 is a view for explaining control of the liquid amount due to thechange in the amount of machining;

FIG. 13 shows a shape of the workpiece when monitored waveforms of FIGS.14, 15 and 19 have been acquired;

FIG. 14 is a monitored waveform for machining voltage, machiningcurrent, discharge pulse number, machining speed, and quiescent timeimmediately after commencement of machining when the shape of FIG. 13has been machined;

FIG. 15 is a monitored waveform for machining voltage, machiningcurrent, discharge pulse number, machining speed, and quiescent timeimmediately after the corner has been passed when the shape of FIG. 13has been machined;

FIG. 16 is a monitored waveform for machining voltage, machiningcurrent, discharge pulse number, machining speed, and quiescent timewhen the shape of FIG. 17 has been machined;

FIG. 17 is a view showing an example of the shape of machining;

FIG. 18 is a block diagram showing an essential part of a conventionalcontroller for electric discharge machine; and

FIG. 19 is a monitored waveform for machining voltage, machiningcurrent, discharge pulse number, machining speed, and quiescent timewhen the shape of FIG. 13 has been machined by the conventional electricdischarge machine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the description will be made of the principle of operation of thepresent invention.

FIG. 3 is an explanatory view for illustrating relationship betweenmachining energy supplied and amount of machining in the wire electricdischarge machining, and when a portion indicated by As in the figureand a portion indicated by Δx have been machined, the followingrelational expressions are established:Ps*w=Δs*t*gPx*w=Δx*t*g  (1)

That is, the following is given.Ps/Δs=Px/Δx=t*g/w  (2)where t is plate thickness of the workpiece 5; Ps and Px are a number ofdischarge pulses to be generated per unit time T in the portionsindicated by Δs and Δx in the figure; w is an amount of machining perdischarge pulse; Δs is distance moveable with discharge pulse number Ps;Δx is distance moveable with discharge pulse number Px; and g ismachining groove width. Also, if the amount of machining w per dischargepulse is constant, the discharge pulse numbers Ps and Px show valuesproportional to the amount of machining which is generated within unittime T respectively.

If machining groove width g is assumed to be constant on condition thatplate thickness t remains unchanged, the following expression isestablished:Ps/Px=Δs/Δx  (3)

The above expression means that if a wire electrode can be fed such thata change (Ps→Px) in discharge pulse number per unit time T and a change(Δs→Δx) in amount of feed movement caused thereby satisfy theabove-described expression (3), machining groove width g becomesconstant.

A reference amount of movement Δs per unit time T can be given frompreset feed speed SPD which is used as a reference and is set andinputted, by means of the following expression:Δs=SPD*T  (4)

From the expressions (3) and (4), the amount of movement Δx is given bythe following expression:Δx=SPD*T*(Px/Ps)  (5)

In this respect, the above described expression (5) shows that when afeed speed SPD is set, the amount of movement Δx is determined by Px/Ps.

FIG. 4 represents relationship between expressions (3) and (5) with theamount of movement Δ taken on the abscissa and the discharge pulsenumber P taken on the ordinate. If discharge pulse number Ps which isused as a reference and a preset feed speed SPD which is used as areference are set, an amount of movement Δx can be determined, using theexpression (5), by counting discharge pulse number Px per unit time Twhich changes every moment during machining.

This amount of movement Δx can, from the expression (1), be representedby the following:Δx=(Px*w)/(t*g)

Px*w in this expression is an amount of machining when discharge pulsePx occurs. Since the amount of movement Δx is obtained by dividing theamount of machining by the product of plate thickness of a workpiece andmachining groove width, moving a wire electrode by the amount ofmovement Δx will correspond to moving the wire electrode by an amountmachined by discharge pulse of Px. That is, the expression (5)represents that the amount of movement Δx of the wire electrodecorresponding to the amount of machining through the discharge pulse isgenerated from the discharge pulse number Ps, which is used as areference, and a counted value Px of the discharge pulse number per unittime T.

Generally, the relationship between an amount of movement A anddischarge pulse number P varies dependent upon material of a workpiece,plate thickness t thereof, machining groove width g and the like.

For example, when a plate thickness t of the workpiece and machininggroove width g are made constant, the relationship between an amount ofmovement Δ and discharge pulse number P becomes one as is shown in FIG.5A. The inclination α of the straight line of FIG. 5A corresponds tot*g/w in the expression (2), but since t and g is made constant, theinclination α represents 1/w in the expression (2) In FIG. 5A, hardmetal WC has an inclination larger than die steel. This means that thehard metal is smaller in amount of machining w per discharge pulse thanthe die steel. This coincides with the fact that it is more difficult toprocess a hard metal with electric discharge machining than to process adie steel with electric discharge machining.

Also, when plate thickness t of the workpiece made of the same materialhas been changed with the machining groove width g kept constant, therelationship between the amount of movement Δ and the discharge pulsenumber P becomes that as is shown in FIG. 5B. In this case, since g andw in the expression (2) are constant, the inclination β of the straightline in FIG. 5B represents the plate thickness t of the workpiece.

Further, FIG. 5C shows relationship between a ratio (P/Δ) of thedischarge pulse number P to the amount of movement Δ and the platethickness t of the workpiece with the machining groove width g madeconstant. Since the following expression is established from theexpression (2),t=(w/g)*(P/Δ)the inclination γ of the straight line of FIG. 5C is given by (w/g). Inthis case, since the machining groove width g is made constant, theinclination γ represents an amount of machining w per discharge pulse.That the inclination γ of the aluminum is large while the inclination γof the hard metal WC is smaller in FIG. 5C means that the amount ofmachining w per discharge pulse is large in aluminum and is smaller inthe hard metal WC. This generally coincides with the fact that aluminumis easy to process by electric discharge machining while the hard metalWC is difficult to process by electric discharge machining.

Since the relationship between an amount of movement Δ and dischargepulse number P varies dependent upon material of the workpiece, platethickness t thereof, machining groove width g and the like as describedabove, on controlling the amount of movement Δx of a wire electrode onthe basis of the expression (5), the relationship between the dischargepulse number Ps which is used as a reference and the preset feed speedSPD (=Δ/T) which is used as a reference is determined in advance. Thatis, concerning workpieces made of various materials, by varying platethickness of the workpieces and the diameter of a wire electrode(machining groove width) in various ways, relationship between dischargepulse number P per unit time T and moving speed Δ per unit time T isdetermined, and ratio κ of discharge pulse number P to amount ofmovement Δ is determined byκ32 P/Δ  (6)

in advance as shown in Table 1 to Table 3. In this respect, if κdetermined in advance is multiplied by the preset feed speed SPD, thedischarge pulse number Ps which is used as a reference can bedetermined. TABLE 1 Diameter of Wire Electrode: ø1 Plate thickness PlatePlate Plate Plate thickness thickness Thickness thickness Material 1 2 .. . j . . . N Material 1 κ₁₁₁ κ₁₁₂ . . . κ_(11j) . . . κ_(11N) Material2 κ₁₂₁ κ₁₂₂ . . . κ_(12j) . . . κ_(12N) . . . Material i κ_(1i1) κ_(1i2). . . κ_(1ij) . . . κ_(1iN) . . . Material M κ_(1M1) κ_(1M2) . . .κ_(1Mj) . . . κ_(1MN)

TABLE 2 Diameter of Wire Electrode: ø2 Plate thickness Plate Plate PlatePlate thickness thickness Thickness thickness Material 1 2 . . . j . . .N Material 1 κ₂₁₁ κ₂₁₂ . . . κ_(21j) . . . κ_(21N) Material 2 κ₂₂₁ κ₂₂₂. . . κ₂₂₁ . . . κ_(22N) . . . Material i κ_(2i1) κ_(2i2) . . . κ_(2ij). . . κ_(2iN) . . . Material M κ_(2M1) κ_(2M2) . . . κ_(2Mj) . . .κ_(2MN)

TABLE 3 Diameter of Wire Electrode: ø3 Plate thickness Plate Plate PlatePlate thickness thickness Thickness thickness Material 1 2 . . . j . . .N Material 1 κ₃₁₁ κ₃₁₂ . . . κ_(31j) . . . κ_(31N) Material 2 κ₃₂₁ κ₃₂₂. . . κ_(32j) . . . κ_(32N) . . . Material i κ_(3ii1) κ_(3i2) . . .κ_(3ij) . . . κ_(3iN) . . . Material M κ_(3M1) κ_(3M2) . . . κ_(3Mj) . .. κ_(3MN)

On starting electric discharge machining, κ is read from the abovedescribed Tables on the basis of the material of the workpiece todetermine the discharge pulse number Ps(=κ*SPD) which is used as areference, by multiplying the read κ by the set feed speed SPD. Thus,during electric discharge machining, an amount of movement (relativemovement) of the wire electrode is controlled on the basis of theexpression (5) while detecting discharge pulse number P per unit time T.

In this respect, a plate thickness which is set as a machining conditiondoes not always exist on the above described Tables, but in such a case,κ corresponding to the preset plate thickness can be determined from aknown κ existing on the Tables. For example, when a preset platethickness is between plate thickness 1 and plate thickness 2, κcorresponding to the preset plate thickness can be determined from κ1corresponding to the plate thickness 1 and κ2 corresponding to the platethickness 2 by means of a method such as proportional distribution.Also, κ corresponding to the preset plate thickness can be alsodetermined by obtaining an approximate curve of κ corresponding to theplate thickness from the value of κ corresponding to the plate thickness1 to plate thickness N. Alternatively, it is also possible to set κmanually while referring to the Tables.

Also, even when the material of a workpiece to be processed actually byelectric discharge machining does not exist on the above describedTables, κ can be set with the following concept. That κ is large meansthat a larger discharge pulse number is required to machine the samedistance (amount of movement Δ), as shown in expression (6), or that theamount of machining w per discharge pulse is small. On the contrary,that κ is small means that less discharge pulse number is required tomachine the same distance or that an amount of machining w per dischargepulse is large. Therefore, in a material which is easy to process byelectric discharge machining, κ becomes small, and in a material whichis difficult to process by electric discharge machining, κ becomeslarge. Accordingly, even when the material of a workpiece to be actuallyprocessed by electric discharge machining does not exist on the abovedescribed Tables, in the case where the degree of difficulty inmachining a material is known from the past experiences and the like,material having nearly the same degree of difficulty in machining as thedegree of difficulty in machining of the material can be searched fromthe above described Tables to be set as material of the workpiece.Alternatively, if the degree of difficulty in machining of a workpieceto be actually processed by electric discharge machining is known to bean intermediate between the degree of difficulty in machining ofmaterial A and the degree of difficulty in machining of material B, anintermediate value between κ of the material A and κ of the material Bcan be also manually set.

Next, control of quiescent time is described with reference to FIG. 8.

FIG. 8 is a view showing a machined state of a workpiece 5 and a stateof machining voltage and current between the workpiece 5 and the wireelectrode 4.

In FIG. 8, Px and Px+1 are discharge pulse numbers per unit time T atpositions indicated by Δx and Δx+1 in the figure respectively; Vx andVx+1 are average machining voltage at positions indicated by Δx and Δx+1respectively; Vp is non-load voltage; Ton is current pulse width; Toffis quiescent time; and Tw(x) and Tw(x+1) are average non-load time perunit time T respectively. Also, Ps, Vs, Tw(s) are discharge pulse numberper unit time T which is used as a reference, reference averagemachining voltage, and reference average non-load time, respectively.Px=T/(Tw(x)+Ton+Toff)Px+1=T/(Tw(x+1)+Ton+Toff)Ps=T/(Tw(s)+Ton+Toff)Vx=Vp*Tw(x)/(Tw(x)+Ton+Toff)Vx+1=Vp*Tw(x+1)/(Tw(x+1)+Ton+Toff)Vs=Vp*Tw(s)/(Tw(s)+Ton+Toff)

Further, assuming that Ton<<Tw+Toff, respective Tw+Ton+Toff are replacedwith actual quiescent time τ to thereby pigeonhole the above describedexpressions.Tw(x)+Toff=τx  (7)Tw(x+1)+Toff=τx+1  (8)Tw(s)+Toff=τs  (9)Px=T/τx  (10)Px+1=T/τx+1  (11)Ps=T/τs  (12)Vx=Vp*(τx−Toff)/τx=Vp*(1−Toff/τx)  (13)Vx+1=Vp*(τx+1−Toff)/τx+1=Vp*(1−Toff/τx+1)  (14)Vs=Vp*(τs−Toff)/τs=Vp*(1−Toff/τs)  (15)

Also, respective average machining current Im(s), Im(x), Im(x+1) perunit time T, and average machining current density Id(s), Id(s) andId(s+1) can be obtained by the following expression, where t is platethickness, and d is machining groove width.Im(s)=Ip*Ton*Ps  (16)Id(s)=Im(s)/(t*g)  (17)Im(x)=Ip*Ton*Px  (18)Id(x)=Im(x)/(t*g)  (19)Im(x+1)=Ip*Ton*Px+1  (20)Id(x+1)=Im(x+1)/(t*g)  (21)From the above described expression (2) and the above expressions (14)to (19), the following expressions are established:Δs/Δx=Ps/Px=Id(s)/Id(x)  (22)Δs/Δx+1=Ps/Px+1=Id(s)/Id(x+1)  (23)That is, these expressions (22) and (23) mean that when machining feedis performed on the basis of the expression (5), the average machiningcurrent density per unit time T also increases or decreases.

This will be described below with reference to FIGS. 6A and 6B.

In FIG. 6A, the abscissa represents sludge concentration SC within adischarging gap, the ordinate represents average machining voltage Vm,and the curved line shows relationship between the sludge concentrationSC and the average machining voltage Vm during electric dischargemachining. When the sludge concentration SC starts to become higher, amultiplicity of minute conductive paths are detected through sludge asan inducement to an electric discharge, and the average machiningvoltage Vm is considered to shift along such a curved line as is shownin FIG. 6A.

In FIG. 6B, the abscissa represents sludge concentration SC within adischarging gap, while the ordinate represents discharge pulse number Poccurring per unit time and actual quiescent time τ. The curved line inFIG. 6B shows relationship between sludge concentration SC which changesevery moment during electric discharge machining and discharge pulsenumber P per unit time, and relationship between sludge concentration SCand actual quiescent time τ.

The curved line represents that, when the sludge concentration SC startsto become higher, a multiplicity of minute conductive paths are detectedthrough sludge as an inducement to electric discharge, discharge pulsesapplied increase and actual quiescent time τ decreases. Also, withdecrease in the actual quiescent time τ along this curved line, non-loadtime Tw also becomes shorter, so that concentrated electric discharge isstarted due to the above described peculiarity of generation of electricdischarge, thereby causing wire disconnection, worse surface roughness,and non-uniform groove width.

In a conventional technique, quiescent time Toff is set to a slightlyhigher value in advance to prevent such a worst condition fromoccurring. Further, when concentrated electric discharge occurs due tosmall area of machining at the time of cutting into an end surface of aworkpiece, and when coolant for an electric discharge portion escapes tocause insufficient cooling, quiescent time Toff is set to a slightlyhigher value in advance.

In the present invention, such a problem is solved by automaticallychanging the quiescent time (Toff) so as to prevent discharge pulsenumber from exceeding a limit to increase. This will be described withreference to FIG. 7A.

The abscissa of FIG. 7A represents actual quiescent time τ, and theordinate represents discharge pulse number P per unit time T, andexpressions (11) and (12) are shown in this figure. When machining isperformed on conditions that the expression (5) is satisfied (that is,on condition that plate thickness t and machining groove width g aremade constant), discharge pulse number P per unit time T and actualquiescent time τ change along the solid line of FIG. 7A in accordancewith amount of machining and sludge concentration SC.

Here, explained below is a quiescent time control wherein a referencedischarge pulse number Ps at which optimum discharge pulse density canbe obtained and the actual quiescent time τs at that time at a point Bon the line are set and a discharge pulse number Px+1 which exceeds thereference discharge pulse number Ps occurs.

In order to cause the discharge pulse number Px+1 at point A, whichsatisfies relationship of Px+1>Ps in FIG. 7A, to approach to thereference discharge pulse number Ps, the preset reference quiescent timeToff(s) can be extended as shown in FIG. 9 by difference between theactual quiescent time τx+1 and τs, that is, by the amount by which thenon-load quiescent time Tw is shortened.

FIG. 9 is a view showing voltage waveform between a wire electrode and aworkpiece, and FIG. 9(c) is a voltage waveform at point B in FIG. 7A,which is a reference voltage waveform. Also, FIG. 9(b) is a voltagewaveform at point A in FIG. 7A, which is obtained before the presentinvention is applied. FIG. 9(a) is a voltage waveform at point A in FIG.7A, which is obtained when the present invention is applied. As seenfrom FIG. 9, the number of electric discharges is large in FIG. 9(b) butsmall in FIG. 9(a).

If quiescent time to be controlled is Toff(x+1), the followingexpressions are established:τs−τx+1=Toff(x+1)−Toff(s)  (24)∴Toff(x+1)=τs−x+1+Toff(s)  (25)

From expressions (11) and (12), the following expression is established:Toff(x+1)=(1/Ps−1/Px+1)*T+Toff(s)  (26)

That is, to control the quiescent time so as to coincide with point B atwhich optimum discharge pulse density can be obtained is realized bydetermining difference between the reciprocal of the discharge pulsenumber Ps which is used as a reference of point B and the reciprocal ofdischarge pulse number Px+1 at point A every unit time T to extend fromthe reference quiescent time Toff(s) by the difference.

Next, with reference to FIG. 7B, the description will be made ofquiescent time control at point C which is lower than the referencedischarge pulse number Ps, that is, when the discharge pulse number Pxoccurs.

In FIG. 7B, the abscissa represents actual quiescent time τ, and theordinate represents the discharge pulse number P and the averagemachining voltage Vm. The curved line of FIG. 7B shows a transition(expression (10) and expression (12)) of discharge pulse number P when τchanges from τS to τx, and a transition (expression (13) and expression(15)) of an average machining voltage Vm.

Since amount of machining at point C is usually small, a discharge pulsehaving long no-load time Tw(x) with long actual quiescent time occurs asshown in FIG. 10(b). From the above described peculiarity of generationof electric discharge, however, consecutive short discharge pulses innon-load time Tw(x) through sludge might occur to cause wiredisconnection. More specifically, voltage lowers in an instant passingover point C′ and point B′ of the average machining voltage, and as aresult, a phenomenon in which the short discharge pulse in non-load timeTw(x) is supplied to the gap is frequently observed. Therefore, in orderto prevent such short discharge pulse from being supplied to the gap,quiescent time may be extended as shown in FIG. 10(a) in advance suchthat actual quiescent time τx does not become lower than τs even ifaverage machining voltage becomes lower than point B′ during machining.

In other words, if quiescent time required in controlling to makeaverage machining voltage Vm at point B′ and point C′ equal to eachother is assumed to be Toff(x), the following expression will beestablished from expressions (13) and (15). $\begin{matrix}\begin{matrix}{{Vx} = {{{{Vp}^{*}\left( {1 - {{Toff}(x)}} \right)}/\tau}\quad x}} \\{= {Vs}} \\{= {{{{Vp}^{*}\left( {1 - {{Toff}(s)}} \right)}/\tau}\quad s}}\end{matrix} & (27)\end{matrix}$  Accordingly, Toff(x)=Toff(s)*τx/τs  (28)By pigeonholing from the expressions (10) and (12), the followingexpression is established:Toff(x)=Toff(s)*(Ps/Px)  (29)

In other words, quiescent time Toff(x) is obtained by multiplyingreference quiescent time Toff(s) by reciprocal of the ratio of dischargepulse number Px to discharge pulse number Ps which is used as areference, whereby the intended object is achieved. As described above,on the basis of the evaluation function based on the expressions (26)and (29), discharge quiescent time is controlled so as to restrainsurplus energy from being supplied in advance.

In this respect, on controlling discharge quiescent time on the basis ofthe expressions (26) and (29), in the same manner as described above, κwhich is determined in advance concerning various materials, platethickness and diameter of the wire electrode can be used. In this case,discharge pulse number Ps (=κ*SPD) which is used as a reference isdetermined by multiplying the determined κ by preset feed speed SPD, andusing this discharge pulse number Ps, the expressions (26) and (29) willbe operated.

FIG. 10 is a view showing a voltage waveform between a wire electrodeand a workpiece, and FIG. 10(c) shows a voltage waveform at point B inFIG. 7B, which is a reference voltage waveform. Also, FIG. 10(b) shows avoltage waveform at point C in FIG. 7B, which is a waveform obtainedbefore the present invention is applied. FIG. 10(a) shows a voltagewaveform at point C in FIG. 7B, which is a waveform obtained when thepresent invention is applied.

At the commencement of machining and in machining of a corner portion inwhich idle feeding occurs, electric discharge is difficult to occur(non-load quiescent time Tw is large) even if voltage is applied betweena wire electrode and a workpiece, and discharge pulse number Px issmaller than the reference pulse number Ps (that is, Ps/Px>11). For thisreason, quiescent time Toff(x) to be determined by expression (29)becomes larger than the reference quiescent time Toff(s). However,since, during that time, the wire electrode relatively moves to theworkpiece to reduce the gap, the non-load quiescent time Tw becomessmaller, electric discharge occurs earlier, and discharge pulse numberPs within the unit time T increases. If discharge pulse number Psincreases, quiescent time Toff (x) to be determined by expression (29)will become shorter to approach to the reference quiescent time Toff(s).

When discharge pulse number Px+1 exceeds the reference pulse number Ps,a quiescent time Toff(x+1) is determined by operating the expression(26), and this quiescent time Toff(x+1) becomes longer than thereference quiescent time Toff (x+1) by T/Ps-T/Px+1 (>0). The longer thequiescent time Toff(x+1) is, the smaller the discharge pulse number Px+1becomes to be (assuming that non-load quiescent time Tw is constant, thelonger the quiescent time Toff(x+1) is, the smaller discharge pulsenumber Px+1 becomes to be).

As described above, quiescent time Toff(x) is controlled such thatdischarge pulse number Px coincides with the reference pulse number Ps.

On the other hand, if discharge pulse number fluctuates, the amount ofmachining to be machined by the electric discharge and the temperaturerise will fluctuate. Thus, the present invention controls temperaturerise in a gap associated with fluctuations in discharge pulse number perunit time T, and controls an amount (flow rate) of coolant (machiningfluid) for discharging sludge which is generated by machining. In otherwords, when discharge pulse number per unit time increases and an amountof machining is large, an amount of coolant will be increased torestrain temperature of the gap from rising, so that sludge will besmoothly excluded. Also, when an amount of machining is small anddischarge pulse number per unit time T is small, an amount of coolant isreduced to prevent supercooling, so that vibration of a wire electrodeis restrained to stabilize electric discharge.

FIGS. 11 and 12 are explanatory views for illustrating relationshipbetween a state of machining and the amount of coolant. From thisfigure, the following relationship is established:Ps∝Qs/w  (30)Px∝Qx/w  (31)Qx/FRx∝Qs/FRs  (32)where w is an amount of sludge per discharge pulse; Qs and Qx areamounts of sludge to be removed by the discharge pulse number Ps and Px;and FRs and FRx are amounts of liquid at the discharge pulse number Psand Px respectively.

From the above described expressions (30), (31) and (32), the followingexpression is established:FRx∝FRs*(Px/Ps)  (33)

In other words, control of an amount of liquid in accordance with anamount of sludge can be achieved by making an evaluation function whichwill become a value obtained by multiplying a preset amount of liquidFRs which is used as a reference, by a ratio of the discharge pulsenumber Ps which is used as a reference to a discharge pulse number Px ata time of change so as to change an amount of liquid FRx.

In this respect, when changing an amount of liquid FRx on the basis ofthe expression (33), κ which has been determined in advance on the basisof various materials, plate thickness and diameter of the wire electrodecan be used in the same manner as described above. That is, a dischargepulse number Ps (=κ*SPD) which is used as a reference is determined bymultiplying the determined κ by the preset feed speed SPD, and usingthis discharge pulse number Ps, the expressions (33) will be operated.

Next, embodiments of the present invention are described on the basis ofthe principle of the present invention explained above.

An essential portion of a controller for a wire electric dischargemachine according to a first embodiment of the present invention isdescribed with reference to the block diagram of FIG. 1. In thisrespect, in FIG. 1, elements identical to those in the conventionalexample shown in FIG. 18 are designated by the identical referencenumerals.

In FIG. 1, reference numeral 1 denotes a discharge pulse generator forgenerating discharge pulse current, composed of: a circuit composed ofan active element such as a transistor for generating discharge pulsecurrent; a charge/discharge circuit for a capacitor; a DC power source;and the like. One of the outputs from the discharge pulse generator 1 isconnected to current-supply brushes 3 located up and down, while theother is connected to a workpiece 5, so that discharge pulse current issupplied between a wire electrode 4 traveling and the workpiece 5.Reference numeral 2 denotes a detection voltage generator composed of: acircuit consisting of an active element such as a transistor fordeveloping detection voltage for detecting a state of the gap, resistor,a capacitor and the like; a DC power source; and the like. One of theoutputs from a detection voltage generator 2 is connected to theworkpiece 5, and the other is connected to the current-supply brushes 3located up and down. A table (not shown) on which the workpiece 5 ismounted is controlled and driven by a X-axis driving motor controller10, a Y-axis driving motor controller 11 and a feed pulse distributiondevice 12 which constitute moving means.

Reference numeral 6 denotes a discharge gap detection device fordistinguishing whether or not a gap is in a dischargeable state in termsof detection voltage. One of the inputs to the discharge gap detectiondevice 6 is connected to the workpiece 5, while the other is connectedto the current-supply brushes 3 located up and down. Thus, whendetermined to be dischargeable, a discharge pulse supply signal isoutputted to the discharge pulse generator 1. At the same time, it isalso outputted to a discharge pulse number counting device 7.

On the basis of a signal outputted from an arithmetic clock 14 everyunit time (predetermined period) T, the discharge pulse number countingdevice 7 counts a discharge pulse supply signal during the period, inother words, substantially counting electric discharge pulse generatedbetween the wire electrode 4 and the workpiece 5.

Reference numeral 8 denotes a reference discharge pulse number memorydevice for storing a discharge pulse number Ps which is used as areference to be inputted in advance. A discharge pulse number comparisonjudging device 9 compares, every unit time T, discharge pulse number Pxwhich the discharge pulse number counting device 7 counted and storedevery unit time T, with a reference discharge pulse number Ps which hasbeen inputted from the reference discharge pulse number memory device 8and stored in advance, to output a ratio (Px/Ps) of the discharge pulsenumber Px to the reference discharge pulse number Ps to a feed pulsearithmetic unit 13, a discharge quiescent time controller 16 and aliquid amount controller 17.

On the basis of a signal transmitted from the arithmetic clock 14 everypredetermined period T, the feed pulse arithmetic unit 13 multiplies adistance (=SPD*T) to be determined from feed speed SPD transmitted fromthe feed speed setting means 15 and predetermined period T by the ratio(Px/Ps) of discharge pulse number Px to the reference discharge pulsenumber Ps transmitted from the discharge pulse number comparison judgingdevice 9 to determine an amount of movement (distance) Δx. In otherwords, the expression (5) will be operated to determine an amount ofmovement Δx, and a pulse train corresponding to this amount of movementΔx is outputted to a feed pulse distribution device 12.

The feed pulse distribution device 12 distributes, from this pulsetrain, driving pulses for X-axis and Y-axis to an X-axis driving motorcontroller 10 and a Y-axis driving motor controller 11 in accordancewith a machining program and drives the X-axis motor and the Y-axismotor for driving the table on which a workpiece has been mounted,respectively.

In accordance with the ratio (Px/Ps) outputted from the discharge pulsenumber comparison judging device 9, a discharge quiescent timecontroller 16 operates the expression (29) when Px≦Ps, or operates theexpression (26) when Px>Ps, to determine a quiescent time Toff foroutputting to the detection voltage generator 2. The detection voltagegenerator 2 will apply voltage between the wire electrode 4 and theworkpiece 5 after this quiescent time Toff is elapsed. In this manner,discharge quiescent time is controlled on the basis of an evaluationfunction which has been set so as to restrain surplus supply of energyin advance.

Also, on the basis of the ratio (Px/Ps) of the discharge pulse number Pxto the reference discharge pulse number Ps outputted from the dischargepulse number comparison judging device 9, a liquid amount controller 17controls an amount of liquid according to such an evaluation function asis shown in expression (33).

As described above, on the basis of the ratio (Px/Ps) of discharge pulsenumber Px to the reference discharge pulse number Ps, and the like, amoving distance, a quiescent time and an amount of coolant arecontrolled every predetermined time, so that surplus supply of energy isrestrained, whereby it is possible to improve machining accuracy as wellas the machining speed.

An essential portion of a controller for a wire discharge machineaccording to a second embodiment of the present invention is describedwith reference to the block diagram of FIG. 2. Here, only portionsdifferent from the first embodiment shown in FIG. 1 will be described.

In this second embodiment, integrated value of discharge pulse currentis obtained by a current detection circuit 18 and a discharge pulsecurrent integrated value computing and storing device 19, instead ofcounting and storing discharge pulse number, and a reference dischargepulse current integrated value storing device 20 is provided, instead ofstoring a reference discharge pulse number pulses. And, a dischargepulse current integrated value comparison judging circuit 21 obtains andoutputs the ratio for controlling feed pulse, discharge quiescent timeand amount of liquid.

That is, a discharge pulse current integrated value which is used as areference can be set, instead of a discharge pulse number Ps which isused as a reference, and a discharge pulse current integrated valueduring machining can be obtained, instead of counting a discharge pulsePx per unit time T which changes every moment during machining. Also, inthe same manner as in the case where κ is determined, values which areused as a reference may be obtained with respect to various materials,plate thickness and diameter of the wire electrode in advancerespectively so that these values can be utilized.

The second embodiment is different from the first embodiment in thatdischarge pulse current integrated value is used in place of dischargepulse number, but is the same as the first embodiment in other respectsincluding operation and effects.

FIG. 14 shows monitored waveforms when a cut has been made into aworkpiece (material: SKD11) with plate thickness of 60 mm from the endsurface using a wire electrode of φ0.2 mm (material: brass).

In FIG. 14, the abscissa represents machining elapsed time (10seconds/division), the left-side ordinate represents machining voltage(V), quiescent time (μsecond), and discharge pulse number per unit time,and the right-side ordinate represents machining speed (mm/minute) andmachining current (A). Hereinafter, the description will be made inaccordance with the time elapsed for machining.

Until lapse of 18 seconds since monitoring is started, the machiningvoltage remains at 70V, the quiescent time at 12 μsecond, the dischargepulse number at 0 level, the machining current at 0 A, and the machiningspeed at 3.5 mm/minute, and the machining feed is being performed inaccordance with the conventional method. From lapse of 18 seconds to 20seconds, the machining voltage lowers to 60V or less, and the machiningcurrent starts to flow. This point of time is detected to start feedingto which the present invention is applied. As an amount of machiningincreases, discharge pulse number also gradually increasescorrespondingly, and at the same time, the quiescent time graduallyapproaches to the preset quiescent time. The machining speed graduallyincreases from about 1 mm/minute, and reaches substantially the targetmachining performance level when 60 seconds are elapsed. From thisresult, it can be confirmed that disconnection based on concentratedelectric discharge which causes a problem when making a cut into the endsurface, insufficient amount of liquid to the discharged portion causedby escape of coolant and the like can be avoided.

FIG. 15 shows monitored waveforms at the passage of the corner portionwhen a workpiece is cut out by machining into the shape as shown in FIG.13. The wire electrode has φ0.2 mm (material: brass), and the workpiecehas plate thickness of 60 mm (material: SKD11). In FIG. 15, the abscissarepresents a machining elapsed time (10 seconds/division), the left-sideordinate represents machining voltage (V), quiescent time (μsecond), anda discharge pulse number per unit time, and the right-side ordinaterepresents machining speed (mm/minute) and machining current (A).Hereinafter, the description will be made in accordance with the timeelapsed for machining.

The waveforms show that the wire electrode does not reach the rightangle corner until elapse of 26 seconds since monitoring is started. Themachining voltage remains at about 42V, the quiescent time at about 12μsecond, the discharge pulse number at about 30, the machining currentat about 3.6 A, and the machining speed at about 1.5 mm/minute.

From the time when the electrode enters the right angle corner with thelapse of 26 seconds until the time when the electrode moves a distancecorresponding to a gap width with the lapse of 32 seconds, the machiningvoltage remains at about 50V, the quiescent time at about 20 μsecond,the discharge pulse number at about 20, the machining current at about 2A, and the machining speed at about 1 nm/minute, and the energy given iscaused to lower.

Thereafter, the speed is gradually increased and, when about 60 secondsare elapsed, the machined state returns to the substantially same stateas that in the portion immediately before the corner. It can beconfirmed that problem at the corner which would occur in the case ofprior art, that is, concentrated electric discharge due to an excessivefeed and excessive energy supply, is avoided.

FIG. 16 shows monitored waveforms which have been obtained bycontinuously machining a workpiece (material: die steel) with platethickness of 60 mm in a zigzag shape having one side shown in FIG. 17 of10 mm and a pitch of 1.5 mm over about one hour using a wire electrodeof φ0.2 mm (material: brass) and monitoring a machining voltage, amachining current, a quiescent time and a machining speed. In FIG. 16,the abscissa represents time elapsed for machining (2 minutes/division),the left-side ordinate represents machining voltage (V) and quiescenttime (μsecond), and the right-side ordinate represents machining speed(mm/minute) and machining current (A).

The explanation of the waveforms will be made below in accordance withthe elapse of machining time. Since the machining is started withcutting into an end surface, quiescent time is extended to about 20μseconds by the control according to the present invention, andaccordingly, machining current and machining speed are also controlledrespectively. The machining speed becomes the largest at a point distantfrom the corner by about 5 mm. At that time, the machining voltage isabout 35V, the quiescent time is about 10 μsecond, the machining currentis about 6.7 A, and the machining speed is about 2.6 mm/minute.Immediately after the passage of the corner, the machining voltage risesto about 47V by the control according to the present invention, but themachining speed lowers to about 1.5 mm/minute, and the machining currentlowers to about 3.6 A.

As described above, it has been confirmed that the machining over a longperiod of time according to the present invention could solve such aproblem of prior art as disconnection immediately after cutting into anend surface of a workpiece and immediately after the passage of thecorner.

Since machining feed corresponding to discharge pulse number can berealized according to the present invention, the following variouseffects are obtained:

-   -   1. It becomes possible to maintain an optimum machining current.    -   2. Wire disconnection which tends to occur when starting to cut,        immediately after the passage of a corner and the like, in which        an amount of machining varies, can be avoided.    -   3. Machining speed with high wire tensile force is increased to        a large extent.    -   4. Machining precision at a corner portion is improved.    -   5. Variation in allowance for machining enlargement is reduced.

1. A controller for a wire electric discharge machine performingelectric discharge machining by applying electric discharge pulsecurrent between a wire electrode and a workpiece while said wireelectrode and said workpiece are caused to relatively move to eachother, comprising: discharge pulse current integrated value computingmeans for computing to integrate discharge pulse current applied everypredetermined time; moving means for relatively moving said wireelectrode and said workpiece to each other along a machining path on thebasis of a moving command; reference discharge pulse current integratedvalue memory means for storing a time integrated value for dischargepulse current which is used as a reference; means for determining aratio of a numerical value obtained by said discharge pulse currentintegrated value computing means to a numerical value stored in saidreference discharge pulse current integrated value memory means; andmeans for outputting, to said moving means, distance obtained bymultiplying relative moving distance between said wire electrode andsaid workpiece to be determined by preset feed speed and saidpredetermined time by said ratio as a moving command every saidpredetermined time.
 2. A controller for a wire electric dischargemachine performing electric discharge machining by applying electricdischarge pulse current between a wire electrode and a workpiece whilesaid wire electrode and said workpiece are caused to relatively move toeach other, comprising: discharge pulse current integrated valuecomputing means for computing to integrate discharge pulse currentapplied every predetermined time; moving means for relatively movingsaid wire electrode and said workpiece to each other along a machiningpath on the basis of a moving command; reference discharge pulse currentintegrated value memory means for storing a time integrated value fordischarge pulse current which is used as a reference; means forcomparing a numerical value obtained by said discharge pulse currentintegrated value computing means every predetermined time with anumerical value stored in said reference discharge pulse currentintegrated value memory means; and a quiescent time controller forcontrolling discharge quiescent time so as to restrain surplus supply ofenergy in accordance with said comparison result.
 3. A controller for awire electric discharge machine performing electric discharge machiningby applying electric discharge pulse current between a wire electrodeand a workpiece while said wire electrode and said workpiece are beingcaused to relatively move to each other, comprising: discharge pulsecurrent integrated value computing means for computing to integratedischarge pulse current applied every predetermined time; moving meansfor relatively moving said wire electrode and said workpiece to eachother along a machining path on the basis of a moving command; referencedischarge pulse current integrated value memory means for storing a timeintegrated value for discharge pulse current which is used as areference; means for determining a ratio of a numerical value obtainedby said discharge pulse current integrated value computing means everypredetermined time to a numerical value stored in said referencedischarge pulse current integrated value memory means; and a liquidamount controller adapted to increase or decrease an amount of coolantin accordance with said ratio.
 4. A controller for a wire electricdischarge machine performing electric discharge machining by applyingelectric discharge pulse current between a wire electrode and aworkpiece while said wire electrode and said workpiece are caused torelatively move to each other, comprising: discharge pulse currentintegrated value computing means for computing to integrate dischargepulse current applied every predetermined time; moving means forrelatively moving said wire electrode and said workpiece along amachining path on the basis of a moving command; reference dischargepulse current integrated value memory means for storing a timeintegrated value for discharge pulse current which is used as areference; and comparison means for comparing a numerical value obtainedby said discharge pulse current integrated value computing means everypredetermined time with a numerical value stored in said referencedischarge pulse current integrated value memory means; wherein on thebasis of the comparison result by said comparison means, dischargequiescent time is controlled and an amount of movement every saidpredetermined time in a moving command to be outputted to said movingmeans is controlled.